I'm using the 19.99 plan myself, it does 10 email addresses and all data. The BB Voice enhancement would give you Tzones, which if you use their proxy would give you full internet; Not all programs support proxy's though in which case those wouldn't run and AFAIK you won't get push email.
A while back, before I owned any Blackberry, I did the whole proxy thing. It worked but since I tether alot it became a royal pain since I'd have to setup the browser each time to go back and forth between using the proxy when on tmobile and not using the proxy when on dsl. There were a few things that didn't work either, IRC I think was one of them.
Personally, it's still not to bad... 39 for 1000 minutes, free night/weekend, and 19 for unlimited internet which I do 4-5 gigs a month. It's still less than I had with Verizon; 80 a month and they would call me most months about using too much "unlimited" internet.